This invention relates to the field of catalytic oxidation of hydrocarbons to produce oxygenated hydrocarbons and more particularly to a method for improving the control of a process for the preparation of maleic anhydride.
Conventionally, maleic anhydride is manufactured by passing a gas comprising n-butane and oxygen through a fixed catalyst bed tubular plug flow reactor containing a catalyst that consists of mixed oxides of vanadium and phosphorus. The catalyst may contain minor amounts of promoters or activators such as iron, lithium, zinc, chromium, uranium, tungsten, various other metals, boron and/or silicon. The oxidation reaction is highly exothermic. Conventionally a shell and tube heat exchanger is used as a reactor with the catalyst packed in the tubes through which the reactant gases are passed. A cooling fluid, typically molten salt, flows over the outsides of the tubes. Because the length to diameter ratio of the tubes is high, the reaction system approaches plug flow. The cooling capacity is substantially uniform throughout the reactor, but the rate of reaction varies widely with the concentration of hydrocarbon reactant and temperature. Because the reactant gases are normally introduced into the catalyst bed at a relatively low temperature, the reaction rate is low in the region immediately adjacent the inlet. However, once the reaction begins, it proceeds at a rapid pace which is accelerated by an increase in temperature resulting from the release of reaction heat. The temperature continues to increase with distance along the length of the reactor tube until a point is reached at which depletion of the hydrocarbon causes the rate of generation to slow, allowing the remainder of the reactor to operate at a lower temperature differential. Thus, a point of maximum temperature is reached, which is generally referred to as the "hot spot" of the reactor.
Problems occur in the operation of the reactor if the hot spot temperature becomes too high. In particular, the selectivity of the catalyst varies inversely with the reaction temperature while the rate of reaction varies directly. Thus, the higher and sharper the hot spot, the greater the proportion of n-butane feed that is consumed by reaction at high temperature and low selectivity. Yield of maleic anhydride may thus be adversely affected. Moreover, exposure of the catalyst bed to excessive temperatures tends to degrade the catalyst, reducing the productivity of the plant and, in some instances the inherent selectivity of the catalyst, i.e., the selectivity at a given reaction temperature. Moreover, because the reaction rate constant increases exponentially with temperature, reaction can run away if the gas temperature substantially exceeds a temperature 80.degree. C. higher than the cooling fluid. Additionally, higher temperatures tend to favor the complete oxidation of the hydrocarbon to CO.sub.2 and water. This not only reduces the yield and productivity of desired product, but higher heat of reaction released in conversion to CO.sub.2 causes the problem to be compounded by further increasing the temperature.
It is known in the art to modulate catalyst activity and enhance selectivity by adding a small proportion of phosphorus compound to the feed gas entering the tubular reactor. Although the function of the phosphorus compound is not fully understood, it is believed that a portion of the phosphorus compound may be sorbed by the catalyst, thereby increasing or restoring the phosphorus/vanadium ratio in the catalyst to a level most favorable for catalyst selectivity. It is believed that phosphorus is lost from the catalyst composition under the catalytic oxidation conditions, and that addition of phosphorus compound may tend to restore the P/V ratio to a desired level which favors formation of maleic anhydride in preference to various by-products.
Incorporation of moisture in the feed gas to the maleic anhydride reactor is also known in the art. Again, the function of this additive is not entirely understood. However, it has been suggested among other things that the incorporation of moisture promotes a relatively even distribution of sorbed phosphorus compound throughout the catalyst bed. In the absence of moisture, it has been observed that the phosphorus compound added to the feed gas tends to deposit in a zone immediately adjacent the inlet to the tubular reactor.
Kerr U.S. Pat. No. 3,474,041 describes the addition of an organophosphorus compound for reactivation of mixed vanadium and phosphorus oxide catalyst for the oxidation of butane to maleic anhydride. Various means for introducing the organophosphorus compound into the catalyst bed are described, including introduction of the phosphorus compound into the butane and oxygen-containing feed gas to the reactor. Best results are said to be obtained by adding the organophosphorus compound after discontinuing hydrocarbon flow and blowing the reactivated catalyst with air prior to the reintroduction of hydrocarbon. A wide range of organophosphorus compounds are said to be useful in the Kerr process. Preferred phosphorus compounds are those wherein the phosphorus has a valence of less than +5, such as phosphines, phosphine oxides, phosphinites, phosphinite esters, the dialkyl phosphites, the trialkyl phosphites, the tetraalkylpyrophosphites, and mixtures thereof. The reference notes that the phosphorus compound can serve as a stabilizer as well as a reactivator for the catalyst.
Click et al. U.S. Pat. No. 4,515,899 describes steam regeneration of phosphorus treated vanadium/phosphorus/oxygen catalyst for maleic anhydride. The reference notes that the treatment of the catalyst with phosphorus compound reduces activity but increases selectivity, the loss of activity being compensated for by an increase in temperature of the reaction. This reference reports that, in practice, it is found that phosphorus compounds concentrate near the feed end of the reactor thus requiring that the amount of phosphorus addition be limited. Addition of steam after treatment with phosphorus compound redistributes the phosphorus compound more evenly through the reaction zone. In one embodiment the phosphorus compound treatment may be conducted over an extended period of time prior to the steam treatment, while in a second embodiment the phosphorus compound treatment and steam treatment may be substantially contiguous, that is the steam treatment follows immediately after each phosphorus treatment. Among the preferred phosphorus compounds used in the Click et al. process are trimethyl phosphite and trimethyl phosphate.
Edwards U.S. Pat. No. 4,701,433 applies both water and phosphorus compound in situ in amounts sufficient to partially deactivate a portion of the catalyst. Edwards teaches that addition of the combination of phosphorus compound and water serves to deactivate the region in which the hot spot of the reaction occurs, thereby moving the hot spot downstream and apparently allowing for reactivation of the region in which the hot spot previously occurred. Use of both phosphorus compound and water also makes the temperature profile of the reactor more isothermal, which further increase maleic anhydride yield. A similar disclosure is contained in Edwards U.S. Pat. No. 4,810,803. Both references disclose the use of alkyl phosphites and alkyl phosphates, including trimethyl phosphate, for treatment of the catalyst bed.
Although the beneficial effect on catalyst selectivity and catalyst life which results from incorporating phosphorus compound in the maleic anhydride reactor feed is well known, the literature available to the art has not closely identified the amounts of particular phosphorus compounds which are most advantageously used. If an excessive proportion of phosphorus compound is included in the reactor feed, not only is an excessive cost incurred in the consumption of phosphorus compound but the activity of the catalyst is unnecessarily decreased and yields are adversely affected. Determination of the appropriate concentrations of trimethyl phosphate in the feed gas to the maleic anhydride reactor is complicated where water is also included in the feed gas. Thus, there has been an unsatisfied need in the art for a method for controlling phosphorus compound addition to achieve the substantial benefits thereof without unnecessary sacrifice of productivity or yield.